3.1. Phases, microstructures, EM properties and EMWAPs of 2D/2D RGO/MoSe2 VDWHs
In order to investigate the phases and chemical compositions, the prepared RGO/MoSe2 samples were characterized by XRD, Raman and XPS. As shown in Fig. 2a, it can be found that the obtained RME samples present the similar XRD diffraction patterns, which indicates their same constituent substances. Amongst, the as-prepared RME1, RME2 and RME3 samples display a broad diffraction peak at ca. 20.71°, which should be corresponded to the phase of RGO [35]. And the comparison results indicate that the obtained RME samples exhibit the progressively broad carbon diffraction peak with the amount of GO increasing from 15 mg to 45 mg, which implies the enhanced carbon disorder. Meanwhile, five characteristic diffraction peaks at ca. 2θ = 13.70°, 31.42°, 34.40°, 37.88° and 55.92° can be detected over all the obtained RME samples, which are well matched with the (002), (100), (102), (103) and (112) crystal planes of MoSe2 (JCPDS: 29–0914), respectively. As labeled in Fig. 2b, the Raman spectra reveal that the obtained RME samples present two characteristic peaks located at ca. 1340 cm− 1 (D band) and 1570 cm− 1 (G band), which are ascribed to the disorder structure of carbon and graphite structure of sp2 C atoms, respectively [36]. It is well recognized that the intensity ratio of D and G peak (ID/IG) is used to evaluate the graphitization of carbon materials. It can be found that he obtained RME1, RME2 and RME3 samples display the ID/IG values of 1.25, 1.27 and 1.29, which further demonstrates their improved defective degrees with increasing the GO amount [37]. The increased ID/IG values confirms that much more amounts of GO was reduced to RGO during the reaction process, which implies the generation of MoSe2 with large quantities [37,38]. To further confirm the surface chemical compositions and chemical valence, XPS measurement was performed over RME2 as an example. As shown in Fig. 2c, the O 1s, Mo 3d, Se 3d and C 1s peaks can be clearly seen in the XPS survey spectrum, which indicates the existence of O, Mo, Se and C elements in the obtained RME2 sample. And the high resolution XPS spectrum (as labeled in Fig. 2d) of Mo 3d displays three different peaks at ca. 228.1 eV, 231.2 eV and 235.2 eV. Amongst, the binding energies of ca. 228.1 and 231.2 eV can be assigned to the Mo 3d5/2 and Mo 3d3/2, which suggests the formation of Mo4+ in the obtained sample [39]. And the peak at ca. 235.2 eV can be attributed to Mo6+, which should originate from the surplus molybdate precursor MoO42− [40]. As presented in Fig. 2e, the high resolution XPS spectrum of Se 3d can be divided into two peaks. The peaks at ca. 54.7 eV and 53.8 eV correspond to Se 3d3/2 and Se 3d5/2, which further confirms the presence of Se2− [17]. Equally, as marked in Fig. 2f, the high-resolution XPS spectrum of C 1s can be deconvoluted into four peaks located at ca. 282.2 eV, 284.1 eV, 285.2 eV and 287.9 eV, which belongs to the sp2 C, C = C/C-C, C-O, C = O, respectively [41]. Generally, the XRD, Raman and XPS results indicate that the RME samples are composed of RGO and MoSe2, which present the improved defective degrees with the increased GO amount.
In order to study their microstructures, the as-prepared samples were characterized by TEM characterization. Figure 3 provides the typical TEM images of RME1, RME2 and RME3 samples. As presented in Fig. 3a, we can find out that the obtained RME1 sample displays a typical 2D wrinkled paper-like geometrical morphology. And the closer TEM observation (as shown in Fig. 3b and c) reveals that large quantities of 2D MoSe2 nanosheets are evenly anchored on the surface of 2D RGO, which corroborates a typical 2D/2D RGO/MoSe2 van der waals heterojunction. Furthermore, large quantities of heterointerfaces between 2D MoSe2 and 2D RGO nanosheets can be clearly observed. Similar to the previous conclusion [35,42], the obtained results indicate that the existence of RGO effectively suppresses the stacking of MoSe2, which results in the obtained RGO/MoSe2 to grow as a relatively thin sheet-like geometrical morphology. Equally, similar to REM1, the TEM investigations indicate that the as-prepared REM2 (shown in Fig. 3d-f) and REM3 (shown in Fig. 3g-i) samples also display the 2D/2D VDWHs and thin paper-like geometries, which are composed of 2D MoSe2 and 2D RGO nanosheets. And the TEM observations demonstrate that much more quantities of 2D MoSe2 nanosheets are grown in situ on the surface of RGO when the amount of GO increases from 15 mg to 45 mg. In general, the obtained results reveal that the 2D/2D sheet-like RGO/MoSe2 VDWHs with different contents of RGO can be produced in high efficiency and large scale through our proposed route. According to the previous models and obtained results [8,43], the designed 2D/2D RGO/MoSe2 VDWHs effectively generate the abundant interfaces, which are conducive to aggrandize interfacial polarization and improve the attenuation of incident EM wave.
To investigate their EM properties and EMWAPs. Figure 4 provides the obtained complex permittivity and RL values of RGO/MoSe2 VDWHs. According to the Debye theory, the \(ε ^{\prime}\) and \(ε ^{\prime\prime}\) values can be expressed as follows [44,45]:
$$ε ^{\prime}={ε _\infty }{\text{+}}\frac{{{ε _s} - {ε _\infty }}}{{1+{{(2\pi f\tau )}^2}}}$$
3
$$ε ^{\prime\prime}=\frac{{{\sigma _{{\text{ac}}}}}}{{{\text{2}}\pi f{ε _0}}}{\text{+}}\frac{{{ε _s} - {ε _\infty }}}{{1+{{(2\pi f\tau )}^2}}}2\pi f\tau {\text{=}}{ε ^{\prime\prime}_c}+{ε ^{\prime\prime}_{\text{p}}}$$
4
Where \({\sigma _{{\text{ac}}}}\), \(\tau\), \({ε ^{\prime\prime}_c}\), \({ε ^{\prime\prime}_{\text{p}}}\), \({ε _s}\) and \({ε _\infty }\) are alternative conductivity, relaxation time, conductivity loss, polarization loss, static permittivity and relative dielectric permittivity at the infinite frequency limit, respectively. As shown in Fig. 4a, it can be found that the as-prepared RME1, RME2 and RME3 samples display the decreased
\(ε ^{\prime}\) values in the tested frequency region, which can be attributed to the increased value on basis of Eq. (3). And their corresponding \(ε ^{\prime}\) values are in the range of 7.78–5.55, 15.33–10.13, and 20.89–12.53, respectively. Additionally, the comparison results reveal that the as-prepared RGO/MoSe2 VDWHs present the enhanced \(ε ^{\prime}\) values with increasing the amount of GO from 15 mg to 45 mg, which is mainly ascribed to the increased content of high dielectric RGO [46]. Equally, as presented in Fig. 4b, the \(ε ^{\prime\prime}\) values of RME1, RME2 and RME3 samples are in the range of 2.13–1.01, 5.72–3.80, and 10.65–5.36, respectively. And their \(ε ^{\prime\prime}\) values are as follows: RME1 < RME2 < RME3. Furthermore, one can find that the obtained \(ε ^{\prime\prime}\) values are not fully inversely proportional to. As presented in Eq. (4), the dielectric loss (\(ε ^{\prime\prime}\)) is composed of conduction loss (\({ε ^{\prime\prime}_c}\)) and polarization loss (\({ε ^{\prime\prime}_{\text{p}}}\)), which are mainly related to their conductivity and dipolar relaxation [24,47]. Amongst, the obtained \({ε ^{\prime\prime}_c}\) value is inversely proportional to. Therefore, the obtained \(ε ^{\prime\prime}\sim f\) curves indicate the important role of polarization loss. It can be found that the obtained RGO/MoSe2 VDWHs display the tunable EM parameters, which is good for optimizing the impedance matching characteristics and improving EMWAPs [48]. Based on the measured EM parameters and transmission line theory, the RL values for RGO/MoSe2 VDWHs were obtained. As shown in Fig. 4c-e. the minimum RL (RLmin) value for RME1, RME2 and RME3 samples are − 51.44 dB with the matching thickness (\({d_m}\)) value of 6.29 mm at 15.20 GHz, -65.34 dB with the \({d_m}\) value of 3.39 mm at 6.40 GHz, and − 26.73 dB with the \({d_m}\) value of 1.88 mm at 11.20 GHz, respectively. Besides, as presented in Fig. 4f-h, the statistical RL results reveal that the obtained RME1 sample exhibits the excellent EMWAPs in the frequency region of Ku band, and the as-prepared RME2 and RME3 samples display the extraordinary EMWAPs in the whole tested frequency region including S, C, X and Ku bands. Furthermore, their corresponding EAB values are 2.40 GHz (15.60–18.00 GHz) with the \({d_m}\) value of 5.71 mm, 4.20 GHz (13.80–18.00 GHz) with the \({d_m}\) values of 1.53 mm, and 4.40 GHz (13.60–18.00 GHz) with the \({d_m}\) value of 1.35 mm, respectively. In general, the obtained results indicate that the elaborately designed 2D/2D RGO/MoSe2 VDWHs display very excellent EMWAPs. Especially, as provided in Fig. 4i, the comparison results suggest that the obtained RME2 and RME3 samples simultaneously displays the outstanding comprehensive EMWAPs including strong absorption capabilities, wide absorption bandwidths and thin matching thicknesses, which are evidently superior to the comprehensive EMWAPs of RME1 sample and the previously reported few-layered MoS2 nonasheets and core@shell structure CNTs@MoS2 nanocomposites [16,49].
3.2. Discussions on the difference in EMWAPs of 2D/2D RGO/MoSe2 VDWHs
Based on the obtained results, it is found that the designed 2D/2D RGO/MoSe2 VDWHs present the excellent and adjusted EMWAPs by controlling the amount of GO. In order to understand the difference in EMWAPs, the impedance matching characteristic, EM wave attenuation, conductivity loss and polarization loss capabilities of RGO/MoSe2 VDWHs were studied in details, which can be evaluated by the values of impedance matching ratio (\({\text{Z}}\)), attenuation constant (\(\alpha\)), \({ε ^{\prime\prime}_{\text{p}}}\) and \({ε ^{\prime\prime}_c}\), respectively. According to the previous works [50,51], these physical parameters can be obtained on basis of the following equations:
$$\alpha =\frac{{\sqrt 2 \pi f}}{c}\sqrt {\left( {\mu ^{\prime\prime}ε ^{\prime\prime} - \mu ^{\prime}ε ^{\prime}} \right)+\sqrt {{{\left( {\mu ^{\prime\prime}ε ^{\prime\prime} - \mu ^{\prime}ε ^{\prime}} \right)}^2}+{{\left( {ε ^{\prime}\mu ^{\prime\prime}+ε ^{\prime\prime}\mu ^{\prime}} \right)}^2}} }$$
5
$${\text{Z=}}\left| {{{{Z_{in}}} \mathord{\left/ {\vphantom {{{Z_{in}}} {{Z_0}}}} \right. \kern-0pt} {{Z_0}}}} \right|$$
6
Based on the measured complex permittivity, Eqs. (3) and (4), the \({ε ^{\prime\prime}_{\text{p}}}\) and \({ε ^{\prime\prime}_c}\) values of RGO/MoSe2 VDWHs can be obtained. As presented in Fig. 5a and b, the comparison results suggest that their \({ε ^{\prime\prime}_{\text{p}}}\) and \({ε ^{\prime\prime}_c}\) values are as follows: RME3 > RME2 > RME1, which implies their progressively enhance polarization loss and conductivity loss capabilities [52]. According to the obtained results, the enhanced \({ε ^{\prime\prime}_c}\) values of RME samples are mainly attributed to the increased RGO contents in the obtained samples, which effectively improve their conductivity [53]. Additionally, it can be seen that the as-prepared RME1, RME2 and RME3 samples display the much higher \({ε ^{\prime\prime}_{\text{p}}}\) values than their corresponding \({ε ^{\prime\prime}_c}\) values, suggesting that the polarization loss plays a major role in the dielectric loss [54]. As presented in Fig. 5c, it can be seen that the as-prepared RME samples display the gradually enhanced \(\alpha\) values with increasing the amount of GO from 15 mg to 45 mg, which also confirms their enhanced EM wave attenuation capabilities. Equally, the \({\text{Z}}\) values (as the presented Fig. 5d-f) of RME samples were obtained on basis of EM parameters and Eq. (6). Compared to the obtained RME1 sample, one can find that the \({\text{Z}}\) values of RME2 and RME3 samples are close to 1, indicate their excellent impedance matching characteristics and good entry of incident EM wave into MAMs [55]. Based on the obtained results, one can find that the enhanced EMWAPs of RGO/MoSe2 VDWHs can be ascribed to their superior impedance matching characteristics, polarization loss, conductivity loss and EM wave attenuation capabilities. Furthermore, compared to the previously reported WS2-RGO heterostructures and honeycomb-like MoSe2@RGO composites [28,56], our elaborately designed RGO/MoSe2 VDWHs display the much better comprehensive EMWAPs because of their excellent interfacial effects and impedance matching characteristics.
3.3. Microstructures, phases, EM properties and EMWAPs of 2D/2D sheet-like RGO/MoS2 VDWHs
Based on the obtained results, one can find that the 2D/2D sheet-like RGO/MoSe2 VDWHs can be produced in high efficiency and large scale through our proposed route. Owing to the abundant interfaces, the obtained results demonstrate that the designed RGO/MoSe2 VDWHs present the excellent and adjusted EMWAPs by controlling the amount of GO. Therefore, our findings not only offer a simple route to produce 2D/2D sheet-like RGO-based VDWHs in high efficiency, but also present an effective pathway to greatly strengthen interfacial effect for developing lightweight high-efficiency MAMs. To further verify the universality of our proposed strategy, as mentioned in the experimental section, the investigation on the production of RGO/MoS2 VDWHs were also constructed. In order to confirm their phases and chemical valence states, Fig. 6 present the XRD patterns, Raman and XPS spectra of RM samples. Taking RM2 and RM3 as representatives, as shown in Fig. 6a, it can be seen that obtained RM2 and RM3 samples present the obvious diffraction peaks at ca. 14.16°, 32.68°, 39.34°, 49.79° and 58.42°, which are well matched with the (002), (100), (103), (105), and (110) lattice planes of MoS2 (JCPDS: 37-1492). Similarly, a broad diffraction peak at ca. 22.72° can also detected over the as-prepared RM2 and RM3 samples, which is also assigned to the phase of RGO [57]. Furthermore, the comparison results reveal that the obtained RM samples display the progressively degraded MoS2 and enhanced carbon diffraction intensities when the amount of GO increases from 30 mg to 45 mg, which should be ascribed to the enhanced content of RGO in the obtained samples [58]. To confirm the existence of RGO, the RM samples were characterized by the Raman spectroscopy. As shown in Fig. 6b, four characteristic Raman peaks located at ca.1345 (D band), 1578 cm− 1 (G band), 2704 (2D band) and 2908 cm− 1 (D + D′ band) can be observed over the obtained RM samples, which further verifies the existence of RGO [59]. In addition, the obtained ID/IG values for RM2 and RM3 samples were 1.15 and 1.19, respectively, further demonstrates that the GO was progressively reduced to RGO with increasing the amount of GO from 30 mg to 45 mg, which is consistent with the results of RME samples. To further affirm the composition and chemical valence, taking RM2 as example, the XPS measurement was conducted. As shown in Fig. 6c, the C 1s, O 1s, Mo 3d and S 2p signal peaks can be observed in the XPS survey spectrum, which are consistent with the XRD results. And the high-resolution XPS spectrum of Mo 3d core level can be divided into six different peaks. Amongst, the peaks at ca. 226.1 eV, 228.4 eV and 231.9 eV are assigned to the binding energies of S 2s, Mo 3d5/2 and Mo 3d3/2, which suggests the formation of MoS2 in the obtained sample [60]. Additionally, two weak peaks located at ca. 233.2 eV and 229.7 eV imply the formation of a Mo-C bond, which further confirms the formation of RGO/MoS2 VDWHs [39,61]. Similarly, as shown in Fig. 6e, the high-resolution XPS spectrum of S 2p core level can be fitted by two peaks located at ca.162.5 eV and 161.2 eV related to S 2p1/2 and S 2p3/2, which suggests the presence of S2− in the obtained sample. And the high-resolution XPS spectrum of C 1s core level (as shown in Fig. 6f) can be deconvoluted into three peaks located at ca. 288.3, 285.6 and 284.3 eV, which originate from the C = O, C-O and C-C/C = C bonds in RGO, respectively [42,62].
To confirm their structures, the TEM investigation was also conducted. As presented in Fig. 7a, it can be clearly seen that the obtained RM2 sample also exhibits a typical wrinkled sheet-like geometrical morphology. And the closer TEM observation (as present in Fig. 7b and c) reveals that a certain amount of MoS2 nanosheets are well distributed on the surface of paper-like RGO, which also displays a typical 2D/2D van der Waals heterojunction. Based on the XRD, Raman, XPS and TEM results, one can confirm that the as-prepared RM2 sample is RGO/MoS2 2D/2D van der Waals heterojunction. Furthermore, plentiful heterointerfaces between the 2D MoS2 and RGO nanosheets can also be clearly observed. Equally, as presented in Fig. 7d-e, the TEM investigation of RM3 sample indicates that large quantities of MoS2 nanosheets are well implanted on the surface of RGO, which also displays a typical 2D/2D RGO/MoS2 van der Waals heterojunction. Furthermore, the comparison TEM investigations of RM2 and RM3 samples reveal that much more amounts of MoS2 nanosheets are grown in situ on the surface of RGO when the amount of GO increases from 30 mg to 45 mg, which can provide much more interfaces. Similar to the obtained RGO/MoSe2 VDWHs, the obtained results further demonstrate that our proposed route is universal and propagable, which can also be utilized to produce RGO/MoS2 VDWHs in high efficiency and large scale.
To further confirm their EM properties and EMWAPs, Fig. 8 provides the complex permittivity and RL values of RGO/MoS2 VDWHs. As shown in Fig. 8a, the \(ε ^{\prime}\) values of RM samples are in the range of 7.89–5.83, 15.27–9.77, and 20.89–11.69, respectively. The as-prepared RM samples also display the decreased \(ε ^{\prime}\) values in the tested frequency region owing to the increased value. With increasing the amount of GO from 15 mg to 45 mg, the as-prepared RM samples present the enhanced \(ε ^{\prime}\) and \(ε ^{\prime\prime}\) (as presented in Fig. 8b) values, which is same to the obtained RME samples. As provided in Fig. 8c and d, it can be found that the RLmin and EAB values for RM1 are − 32.55 dB at 12.6 GHz with the \({d_m}\) value of 7.26 mm, and 2.60 GHz (15.40–18.00 GHz) with the \({d_m}\) value of 5.59 mm, respectively. Equally, the as-prepared RM2 sample displays RLmin value (Fig. 8e) of -50.94 dB at 5.4 GHz with the \({d_m}\) value of 3.95 mm, and EAB value (Fig. 8f) of 4.20 GHz (13.80–18.00 GHz) with the \({d_m}\) value of 1.55 mm, respectively. And the RLmin and EAB values (Fig. 8g and h) for RM3 are − 26.10 dB at 16.4 GHz with the \({d_m}\) value of 1.34 mm, and 4.20 GHz (13.80–18.00 GHz) with the \({d_m}\) value of 1.41 mm, respectively. As summarized in Fig. 8i, it can be seen that the as-prepared RM2 and RM3 samples also display the superior EMWAPs compared to RM1 sample, which also further verifies the previous result. Additionally, the obtained results also suggest that the as-prepared RME2 and RME3 samples simultaneously display the outstanding comprehensive EMWAPs including strong absorption capabilities, wide absorption bandwidths and thin matching thicknesses, which are same to the obtained RGO/MoSe2 VDWHs. In general, the obtained EM properties and EMWAPs of RGO/MoS2 VDWHs are well consistent with the results of RGO/MoSe2 VDWHs, which further demonstrates the good repeatability of the designed experiment and our ideas.
3.4. Analyses on the difference in EMWAPs and main avenues of EM wave attenuation
To understand the influence of GO amount on the EMWAPs, Fig. 9 provides the \({ε ^{\prime\prime}_c}\), \({ε ^{\prime\prime}_{\text{p}}}\), \(\alpha\) and \(\tan {\delta _ε }={{ε ^{\prime\prime}} \mathord{\left/ {\vphantom {{ε ^{\prime\prime}} {ε ^{\prime}}}} \right. \kern-0pt} {ε ^{\prime}}}\) values of obtained RGO/MoS2 VDWHs. As provided in Fig. 9a and b, it can be found that the as-prepared RGO/MoS2 VDWHs display the progressively enhanced \({ε ^{\prime\prime}_{\text{p}}}\) and \({ε ^{\prime\prime}_c}\) values with increasing the amount of GO from 15 mg to 45 mg, which implies their improved conductivity loss and polarization loss capabilities. Furthermore, the comparison results reveal that the obtained RM samples present the much higher \({ε ^{\prime\prime}_{\text{p}}}\) values than \({ε ^{\prime\prime}_c}\) values, which further affirms the key contribution of polarization loss. Based on the obtained results, one can conclude that the elaborately designed RGO/MoS2 VDWHs generate the abundant interfaces, which result in their greatly improved polarization loss abilities [63]. Equally, same to the obtained RME samples, it can be seen that the RM samples also present the progressively enhanced \(\alpha\) values (as provided in Fig. 9c) when the amount of GO increases from 15 mg to 45 mg, which further verify confirms their improved EM wave attenuation capabilities. As shown in Fig. 9d, the comparison results also reveal that the \(\tan {\delta _ε }\) values of RM samples are as follows: RM3 > RM2 > RM1, which also demonstrates the improved dielectric loss capabilities. In general, the obtained results indicate that the increased amount of GO in the designed RGO/MoS2 VDWHs effectively produces the abundant interfaces and improves the conductivity, which leads to their boosted polarization loss, conductivity loss and dielectric loss capabilities.